1. Field of the Invention
This invention concerns a photo semiconductor integrated circuit device having a photodiode portion and an amplifier portion.
2. Description of the Related Art
A photo semiconductor integrated circuit device having a photodiode portion and an amplifier portion is used for light detection and signal processing, for example, in CD (Compact Disk) drives or DVD (Digital Versatile Disk) drives as optical information recording reproducing apparatus. A semiconductor integrated circuit device and a photodetector have been generally manufactured separately so far and detection signals from a photodiode are sent by way of wirings such as lead wires to the semiconductor integrated circuit device and applied with processing such as amplification. However, in the CD drives, it has been demanded for high-speed operation of reading and size reduction of apparatus, and those referred to as OEIC (Optoelectronic Integrated Circuit Device) in which a photodiode and a semiconductor integrated circuit are prepared on one identical substrate have been manufactured in order to cope therewith. The structure is described for example in JP-A-266033/1999. Further, JP-A-82268/1992 describes a semiconductor device having a semiconductor substrate of a first conduction type and a photodiode constituted with an epitaxial layer of a second conduction type in which a semiconductor region at a concentration lower than an epitaxial layer or the semiconductor surface is formed below the epitaxial layer, or a semiconductor region of a first conduction type at a concentration higher than the semiconductor substrate is formed below the epitaxial layer thereby improving the responsivity and extending the band width of the photodiode.
FIG. 2 is a schematic cross sectional view for one example of a photo semiconductor integrated circuit device with a photodiode prepared on an SOI (Silicon on Insulator) substrate. In FIG. 2, are shown a photodiode portion 1 and a transistor portion 2 as a part of an amplifier portion. The devices are prepared on the SOI substrate in which an n-type silicon handle wafer 30, an oxide layer 40 and a silicon crystal layer (that is, an SOI layer 31) are formed.
In the transistor portion 2, a collector 63, an emitter 64 and a base 65 are formed on a passivation layer 43. An nxe2x88x92 type epitaxial layer 32 is present on the SOI layer 32, and constitutes together with a base diffusion layer 33 and an emitter diffusion layer 35. A polysilicon layer 34 is provided for leading out the base layer 33, and an oxide layer 45 is provided on layer 34. A buried layer 50 as a high concentration impurity layer prepared on the surface of the substrate before growing a silicon layer by epitaxial growing, is formed by introducing an impurity into the SOI layer 31 for lowering the collector resistance and is connected by way of an n-type diffusion layer 51 for collector junction to an upper electrode (collector). It is conducted with the emitter 64 by way of the emitter diffusion layer 35, polysilicon 36 for emitter and a silicide layer 66. A side wall oxide layer 42 insulates polysilicon for the emitter and the base. Devices are separated from each other by an inter-device isolating buried oxide layer 41 and intra-device isolation is attained by buried oxide layer 46 as a shallow groove.
In the photodiode portion 1, are shown a cathode electrode 61 and an anode electrode 62 of the photodiode. Light 10 to be detected transmitting the oxide layer 44 generates carriers in a p+ layer 37, an epitaxial layer 32, and a buried layer 50 to form an photo current between the electrodes 61 and 62. A polysilicon layer 34 is provided for leading out the p+ layer 37. The photodiode has a buried layer 50 as in the case of the transistor and is connected to the cathode electrode (upper electrode) 61 by way of an n-type diffusion layer 52 and a suicide layer 67 for cathode connection. Although not illustrated in this example, current from the photodiode is put to signal processing by a group of transistor integrated circuits.
In the photodiode portion 1, are shown a cathode electrode 61 and an anode electrode 62 of the photodiode. Light 10 to be detected transmitting the oxide layer 44 generates carriers in a p+ layer 37, an epitaxial layer 32, and a buried layer 50 to form an photo current between the electrodes 61 and 62. The photodiode has a buried layer 50 as in the case of the transistor and is connected to the cathode electrode (upper electrode) 61 by way of an n-type diffusion layer 52 and a silicide layer 67 for cathode connection. Although not illustrated in this example, current from the photodiode is put to signal processing by a group of transistor integrated circuits.
FIG. 3 shows the change of intensity of light in the inside of silicon when silicon crystals are irradiated. The intensity of light is normalized by the intensity at the surface. While the intensity of light decays as the depth increases from the surface and the state of decay is different depending on the wavelength of light. Near the wavelength at 780 nm used in CD drives, light intrudes deeply as far as the inside of the silicon crystals but light at a shorter wavelength of 410 nm is substantially decayed near the surface. Further, the intrusion state of light at a wavelength of 660 nm used in DVD drives situates between both of them.
The light intruding to the inside of silicon generates carriers to form photocurrent. The relation between the state of generation of carriers and the structure of the photodiode constitutes a factor determining the responsivity of the photodiode and the frequency response. FIG. 3 shows an example of a size for the cross sectional structure of a photodiode. PD layer shows a range from p+ layer at the surface of the photodiode to a depletion layer end including an nxe2x88x92 layer. SOI layer is a silicon crystal layer in which a buried layer is formed. A reverse bias is applied at a sufficient level to the SOI layer relative to the PD layer and the depletion layer reaches as far as the SOI layer. At the wavelength of 410 nm shown by the solid line, since almost of light is absorbed in the PD layer, the cutoff frequency is determined by the drifting speed of the carriers and it is expected to be a cutoff frequency at about Giga Hz. On the other hand, at a wavelength of 780 nm shown by the short dotted line, the light reaches at a sufficient intensity as far as the SOI layer and further reaches as far as the handle wafer. Since a voltage is not applied in the SOI layer as in the depletion layer, the photo generated carriers form a photocurrent through the diffusion process. Since the diffusion process is an extremely slow process, the frequency band width of the photodiode is remarkably narrowed as the ratio of this current increases.
Further, the photodiode responsivity can be improved when more photo generated carriers enter the depletion layer. In the photodiode using the SOI substrate, since it is separated by an insulator from the handle wafer, photo-carriers generated in the handle wafer do not contribute to the photo-current of the photodiode. Accordingly, in a case where a great amount of light intrudes through the oxide layer into the handle wafer as in the case of the light at 780 nm shown in FIG. 3, photodiode of higher responsivity is no more obtainable.
At present, high-speed readout has been demanded in compact disk drives or digital versatile disk drives and higher responsivity and broader band width are required for the photodiode. However, while the production process is optimized to the integrated circuit device portion, when the cutoff frequency or the responsivity of the photodiode is intended to be improved by changing the thickness of the SOI layer and the thickness of the PD layer, the performance of the integrated circuit device portion such as for transistors may possibly be deteriorated.
In view of the foregoing problems, this invention intends to provide a photo semiconductor integrated circuit in which the frequency characteristic or the responsivity of a photodiode prepared on a substrate identical with that for an integrated circuit device is improved without greatly changing the production process for the integrated circuit device, that is, without deteriorating the performance of the integrate circuit device.
In accordance with this invention, the frequency band width of a photodiode is improved by using an impurity distribution or concentration different from that of the buried layer in an amplifier portion is used in a buried layer in a photodiode portion. Further, the responsivity is improved by optimizing the thickness of an insulator of a substrate.
FIG. 4 shows a schematic view for the cross section of a photodiode. An SOI substrate 300 is used and reference numeral 40 denotes an insulator comprising an oxide layer. Reference numeral 32 denotes an epitaxial layer and a p+ layer 37 is formed at the uppermost portion. FIG. 4(A) illustrates a photodiode portion of an existent structure having a buried layer 50 identical with that for an amplifier portion. In the photodiode portion of the structure shown in FIG. 4(B) according to this invention, the impurity concentration in a buried layer 501 is changed by providing a mask different from that for burying in the integrated circuit device portion and changing ion implanting conditions.
It is assumed that the epitaxial layer is completely depleted and carriers by incident light 10 are generated in the epitaxial layer 32 and the SOI layer changed to the buried layer. Since a reverse bias at a sufficient level is applied between the p+ layer buried layer 37 and the SOI layer, a drift current flows to the epitaxial layer 32. On the other hand, since no voltage is applied to the SOI layer, the photo-carriers form a diffusion current. While the diffusion current is a factor of worsening the frequency response of the photodiode, when the impurity concentration in the buried layer 501 is changed, the diffusion constant therein can be changed to change the diffusion rate.
FIG. 5 shows the change of the cutoff frequency (defined as a frequency for 3 dB lowering) depending on the change of the impurity concentration in the buried layer. Calculation was conducted under the conditions assuming, in the cross sectional view of FIG. 4(B), the thickness a of the epitaxial layer as 1.2 xcexcm, the thickness b for the SOI layer as 1.5 xcexcm, the wavelength of light as 780 nm and the minority carrier lifetime in the buried layer as 3xc3x9710xe2x88x923 sec. As shown in FIG. 5, it can be seen that the frequency band width is widened as the carrier concentration is decreased. It is practical to decrease the concentration within about one digit when the impurity concentration in the buried layer of the transistor portion is about at 1xc3x971018 cmxe2x88x923. Even if it is decreased further, the effect of increasing the resistance in the buried layer lowers the cutoff frequency to provide an adverse effect. Accordingly, the impurity concentration in the buried layer of the photodiode portion is preferably about from 1xc3x971017 cmxe2x88x923 to 1xc3x971018 cmxe2x88x923. Further, while the frequency band width is widened as the thickness of the epitaxial layer forming the depletion layer is increased, the thickness of the epitaxial layer is preferably 0.8 xcexcm or more while taking the thickness of the SOI layer into consideration since light intrudes deeply in a case of detecting light at a longer wavelength of 780 nm.
Further, for improving the responsivity of the photodiode, as shown in FIG. 6, it adopts a method of increasing the reflected light by an oxide layer 401 of an SOI substrate thereby generating carriers in the photodiode. When the thickness c for the oxide layer 401 in FIG. 6 is changed, the intensity of the reflected light 11 changes. Since the reflected light 11 generates carriers in the inside of the photodiode again, the responsivity is improved when the reflected light is increased.
The reflectivity R(xcex4) by the oxide layer 401 in the inside of the silicon is represented by the following equation (1).
R(xcex4)=2r2{1xe2x88x92cos(2xcex4)}/(1xe2x88x922r2cos(2xcex4)+r4}xe2x80x83xe2x80x83(1) 
where xcex4=2 xcfx80nc/xcex, r represents a reflection coefficient when a light incidents vertically from silicon to an oxide layer, n represents a refractive index of the oxide layer, c represents the thickness of the oxide layer and xcex represents the wavelength of the light.
It can be seen from the equation (1) that the reflected light increases on the condition that the thickness c of the oxide layer is about: xcex÷(4xc3x97n)xc3x97(positive odd number) Assuming the wavelength used as 780 nm, the surface reflectivity as 0.26 and the refractive index n for the oxide layer as 1.46, first two cases for the oxide layer thickness c that maximize the reflectance are 130 nm and 401 nm according to the equation described above.
Further, FIG. 7 shows the dependence of the responsivity on the thickness of the oxide layer when the same structure as the photodiode used in FIG. 4 is adapted according to the equation described above. The result of calculation for the responsivity shows the same trend and the responsivity is maximized at the maximum reflectance. While calculation has been conducted only up to the layer thickness including the two initial peaks, it is expected for the larger layer thickness that peaks for the responsivity will appear at about: xcex÷(4xc3x97n)xc3x97(positive odd number). As described above, a photodiode of higher responsivity can be obtained by selecting the thickness of the oxide layer of the SOI substrate such that the reflected light is maximized relative to the wavelength used.
Then, as a method of making the cutoff frequency higher, when the buried layer formed in the SOI layer, as shown in the cross sectional structure of the photodiode in FIG. 8, a buried layer 502 is distributed deeper by d from the surface of the SOI layer compared with the buried layer 50 of the existent structure shown in FIG. 4(A). Such a distribution of the impurity can be formed by controlling the ion implantation conditions. As a model for the calculation, it is considered a structure prepared such that the impurity in the buried layer 502 is distributed from the depth at d=0.2 xcexcm with the surface of the SOI layer as a reference toward the oxide layer 40. Other conditions are identical with those for the case of FIG. 4.
The result of calculation is shown in FIG. 9. In FIG. 9, the thickness of the oxide layer is expressed on the abscissa, the cutoff frequency is expressed on the ordinate, a solid line 90 shows the change of the cutoff frequency in a case of implantation at d=0.2 xcexcm and a dotted line 91 shows the change in the existent structure in FIG. 4(A). While the cutoff frequency fluctuates somewhat depending on the change of the thickness of the oxide layer, it can be seen that the solid line 90 having a deep impurity distribution has a high cutoff frequency. While consideration is made to a case where d=0.2 xcexcm, similar result can also be obtained at d of more than 0.2.
The photo semiconductor integrated circuit device according to the invention based on the foregoing study provides a photo semiconductor integrated circuit device having an SOI substrate, an amplifier portion formed on the SOI substrate, and a photodiode portion formed on the SOI substrate, wherein each of the amplifier portion and the photodiode portion has a buried layer containing an impurity and being connected with an upper electrode, the junction between a first conduction type and a second conduction type in the photodiode portion is in the inside of an epitaxial layer on the SOI substrate, and the impurity concentration of the buried layer of the photodiode portion is lower than the impurity concentration of the buried layer of the amplifier portion. The thickness of the epitaxial layer is preferably 0.8 xcexcm or more. The upper limit for the thickness of the epitaxial layer is practically about 2 xcexcm considering the problem in view of manufacture of a connection layer for connecting the buried layer and the upper electrode (cathode or collector). Further, the impurity concentration of the buried layer in the photodiode portion is preferably within a range from 1xc3x971017 cmxe2x88x923 to 1xc3x971018 cmxe2x88x923.
By adopting the structure described above, the frequency response of the photodiode prepared on one identical substrate together with other semiconductor integrated circuit device can be improved.
The photo semiconductor integrated circuit device according to this invention also provides a photo semiconductor integrated circuit device having an SOI substrate, an amplifier portion formed on the SOI substrate and a photodiode portion formed on the SOI substrate, wherein the impurity concentration in the SOI substrate of the photodiode portion is lower on the side of the surface of the SOI substrate than the impurity concentration on the side of the insulator of the SOI substrate.
By adopting the constitution as described above, the frequency response of the photodiode prepared on one identical substrate together with other semiconductor integrated circuit device can be improved.
Further, the photo semiconductor integrated circuit device according to this invention also provide a photo semiconductor integrated circuit device comprising an SOI substrate, an amplifier portion formed on the SOI substrate, and a photodiode portion formed on the SOI substrate, wherein each of the amplifier portion and the photodiode portion has a buried layer containing an impurity and being connected with an upper electrode, and the buried layer in the photodiode portion is formed at a position deeper by 0.2 xcexcm or more than the buried layer in the amplifier portion.
By adopting the constitution as described above, the frequency characteristic of the photodiode prepared on one identical substrate together with other semiconductor integrated circuit device can be improved.
Further, the photo semiconductor integrated circuit device according to this invention also provide a photo semiconductor integrated circuit device comprising an SOI substrate, an amplifier portion formed on the SOI substrate, and a photodiode portion formed on the SOI substrate, wherein the thickness of an insulator in the SOI substrate of the photodiode portion is: about xcex÷(4xc3x97n)xc3x97m where the xcex represents the wavelength of an incident light, n represents the refractive index of the insulator and m represents a positive odd number. The thickness for the insulator of the SOI substrate of the photodiode portion is preferably about xcex÷(4xc3x97n)xc3x97m and, practically, it is preferably within a range of: {xcex÷(4xc3x97n)xc3x97m}xc2x1{xcex÷(8xc3x97n)].
By adopting the constitution as described above, the frequency characteristic of the photodiode prepared on one identical substrate together with other semiconductor integrated circuit device can be improved.
The optical recording reproducing apparatus according to this invention comprises an optical disk for recording information, a semiconductor laser light source, an optical system for focusing an emission light from the semiconductor laser light source to the optical disk, a photodetector for detecting a reflected light from the optical disk and a signal processing section for processing signals detected by the photodetector, wherein the photo semiconductor integrated circuit device is used for the detection of the reflected light and the processing of at least a portion of the detected signals.
The optical recording reproducing apparatus can improve the reproducing speed of the optical recording reproducing device by the improvement of the responsivity and extension of the frequency band width of the photodiode of the photo semiconductor integrated circuit device to be used. Further, in a case of detecting lights of different wavelengths, since it has an effect of improving the responsivity and extending the frequency band width to wavelength for which the responsivity is low and the frequency band width is narrow, one photo semiconductor integrated circuit apparatus can cope with multiple wavelength.
Other and further objects, features and advantages of the invention will appear more fully from the following description.